1. Field of the Invention
The present invention relates generally to miniature inductors and a process of making the same.
The invention is more specifically related to forming compact circuits including semiconductor integrated circuits and miniature inductive devices.
2. Description of the Prior Art
Conventional inductors, such as those used in power handling applications and the like, are typically made of discrete magnetic cores and continuous lengths of wire wound about the cores. The integration of such discrete inductors with integrated analog or digital microcircuits has been difficult if not impossible to implement in the past because of incompatibilities between the fabrication processing requirements or operating requirements of integrated circuit chips and those of known inductors.
One incompatibility which has long existed between integrated circuit chips (IC's) and inductors is size difference. Traditional inductors tend to be much larger than IC's. Small, discrete inductors have been recently developed and commercialized by companies such as TDK of Japan, but these discrete devices have not been integrated with semiconductor chips because of other incompatibilities which still remain.
By way of example, U.S. Pat. No. 4,322,698, entitled "Laminated Electronic Parts And Process For Making The Same" and issued Mar. 30, 1982 to Takahashi et al., discloses a method of making a discrete, miniature inductor wherein a monolithic structure is produced using screen printing and ceramic sintering. An insulating sheet composed of magnetic or dielectric material is formed from a paste, and a conductive pattern is deposited thereon. A second sheet of electrically insulating material or electrically insulating magnetic material is superimposed on the first sheet and a second conductive pattern is formed thereon. The second conductive pattern is then electrically connected to the first conductive pattern. The process is repeated until a laminated part having a desired number of alternate layers is obtained. Finally, thin terminal electrodes are attached to edges of the laminated part. Individual layers of the laminated part are produced by a thick film ceramic process wherein powders of selected materials are kneaded with a suitable binder and a suitable solvent to form a paste. The paste is extruded into or stamped out as thick sheets which are about ten microns thick. The sheets are patterned with conductive material and laminated as described earlier and the resulting aggregate structure is sintered to bind the laminations together. Inductors produced by this process are smaller than previous wire wound inductors but this technique is incompatible with integrated circuit technology because there is the danger that the sintering temperatures can destroy the microcircuitry of an IC chip. Thus, the combination of this technique with semiconductor IC's is not possible. Further, since this technique uses thick film technology for patterning the sheets, the resulting inductors tend to be larger in size than inductors produced by thin film techniques. So the Takahashi method does not provide an inductive structure of ultra-miniaturized size, and the idea of integrating such large inductors with ultraminiature IC's does not seem practical.
Unfortunately, the integration of known thin film techniques with IC's is also impractical. Such thin film techniques are incompatible with conventional IC technology for other reasons. By way of one example, K. Kawabe, H. Koyama and K. Shirae, describe in an article entitled "Planar Inductor," IEEE Trans. on Magnetics, Vol. Mag-29, No. 5, September 1986, several types of planar inductors which can be produced by thin film technology. The conductive turns making up an inductor of this technique are all coplanar, i.e., they each lie in one and the same plane, and the plane of the turns is parallel to the substrate. Magnetic flux lines induced by such planar turns tend to flow in a direction perpendicular to the plane of the turns and the plane of the substrate. This orientation can interfere with adjacent semiconductor circuitry if such semiconductor circuitry is provided and brought into close proximity with (i.e. less than 1000 microns away from) the planar inductor. This makes planar inductors incompatible with integrated circuit chips having circuitry disposed at a planar surface thereof.
The process of making planar inductors begins with direct current (D.C.) sputter deposition of permalloy magnetic material on a glass substrate. The magnetic material is capped by an insulating layer of silicon monoxide, the latter being deposited by vacuum evaporation. A thin chromium layer is deposited on the capped magnetic material and a layer of conductive copper is then deposited on the thin chromium layer. A planar coil is patterned in the copper and chromium layers by standard photolithographic and etching steps. After forming the planar coil, a second insulating film of silicon monoxide and a top layer of permalloy material are deposited by vacuum techniques.
Another planar type of thin film inductor is described by N. Saleh and A. H. Qureshi in an article entitled "Permalloy Thin Film Inductors," Electronics Letters, pp. 850-852, Dec. 1970. A planar spiral conductor, composed of copper, is surrounded by a pair of spaced-apart permalloy thin films. The permalloy thin films (1000-3000 .ANG. hick) are formed by vacuum deposition. The conductive copper coil is interposed between the Permalloy thin films by depositing an SiO on bottom permalloy layer then using vacuum evaporation of copper and electroplating of additional copper to obtain 25 .mu.m thick copper conductors. The spacing between turns is 10.sup.-3 inch and the conducting path is 5.times.10.sup.-3 inch wide. The silicon monoxide (SiO) layer is about 10,000 .ANG. thick. The whole pattern lies on a glass substrate.
Yet another method of making small inductors is proposed by R. F. Sooho, in an article entitled "Magnetic Thin Film Inductors For Integrated Circuit Applications", IEEE Trans. on Magnetics, Vol. Mag-15, No. 6, November 1979. The Sooho article states that it is difficult to fabricate small air-core inductors of sufficiently high inductance and Q (the ratio of reactive impedance and conductor resistance) due to the large number of turns that must be realized in an extremely small space. In order to minimize the number of turns for realizing a given inductance, the Sooho article suggests to fabricate thin film inductors with high permeability cores. The inductors are manufactured by forming a permalloy film on a glass or Si substrate. SiO.sub.2 is deposited onto the film using spin-on oxide deposition, after which five turns of wire are deposited so as to wrap them around the combination of the SiO.sub.2, permalloy film and substrate in the manner shown in FIG. 1.
It is observed in the Sooho paper that the achieved inductance and susceptibility are not as high as expected. This is attributed to the fact that the magnetic path is not formed entirely of a permalloy material. The core of the coil has a permeability determined in part by the contributions of the nonpermalloy substrate and the air gap; and is thus, less than that of the permalloy material taken alone.
As can be seen from the above, prior proposals for forming miniature inductors have provided techniques which are incompatible with the formation and/or operation of integrated circuit chips. Problems arise because processing temperatures may be too high, inductor permeability may be too low, and as will now be explained, because it is unacceptable to have a situation where time variant flux lines may pass through sensitive portions of an integrated circuit chip.
When time variant flux lines of substantial strength pass through conductive portions of an integrated circuit, it is possible for such flux lines to induce undesired voltages or currents within the integrated circuit. As a consequence, IC's are usually spaced far apart from magnetic components. The aforementioned planar inductors (i.e., of Saleh et al.) are not integrable with IC chips because their planar orientation tends to generate flux lines that pass orthogonally into the planar circuitry of an IC chip. The aforementioned Si-substrate, thin film inductors of Sooho (FIG. 1) are not integrable with IC chips because flux lines are induced to flow directly through the cross-sectional area of the Si- substrate portion of the coil core.
The above described problems have biased practitioners in the integrated circuits arts away from the use of magnetic components (i.e., inductors, transformers and relays) and towards pseudo-equivalent components which rely on nonmagnetic properties (i.e., capacitors, optical isolation and so-called "solid state relays"). But such pseudo-equivalent components do not always perform as well as their magnetic counterpart. By way of example, it is difficult to produce RC based filters which have the same characteristics as LC based filters. By way of further example, optical circuit isolation does not provide the same coupling efficiency as transformer based isolation. By way of even further example, solid state relays rarely operate in a manner which allows them to be substituted for electromechanical relays (electromagnetic relays). Electromechanical switches or relays are routinely used in a variety of applications such as high-frequency telecommunication network switching, automotive control and large equipment control (i.e., starting large horsepower electric motors). They offer advantages which are not easily replicated by nonmagnetic, solid-state substitutes.
It has been common practice to provide magnetic components, such as transformers and electromechanical relays, as discrete components on printed circuit boards, separate from IC chips. This practice evolves from the incompatibilities between known inductor fabrication techniques and conventional chip fabrication processes. Such discrete types of circuitry tend to have higher cost and lower reliability than integrated circuitry. It would be desirable and advantageous to have one or more methods by which magnetic components such as inductors, transformers and electromechanical relays can be integrally formed with semiconductive microcircuitry.